The subject matter of the present invention relates to a new method and apparatus associated with a Combinable Magnetic Resonance Tool (CMRT) adapted to be disposed in a wellbore for determining the Larmor frequency of the tool in the borehole prior to the use of the tool for logging operations.
Various logging tools adapted to be disposed in a wellbore utilize the principles of nuclear magnetic resonance. One such tool is known as the Combinable Magnetic Resonance Tool (CMRT), also known as the Pulsed Nuclear Magnetism Tool (PNMT). The CMRT tool is described in detail in the following two prior pending patent applications, the disclosures of which are incorporated by reference into the specification of this application: (1) application Ser. No. 07/970,324, filed Nov. 02, 1992, entitled "Borehole Measurement of NMR Characteristics of Earth Formations" by Jason P. Macinnis et al, and (2) application Ser. No. 07/970,332, filed Nov. 02, 1992, entitled "Processing Method and Apparatus for Processing Spin Echo Inphase and Quadrature Amplitudes from a Pulsed Nuclear Magnetism Tool and Producing new Output Data to be Recorded on an Output Record" by Robert Freedman now U.S. Pat. No. 5,291,137. The CMRT tool includes a magnet housing for enclosing a magnet array. The magnet array produces a static magnetic field B0 in the earth formations adjacent to the tool. A nearby antenna produces an oscillating radio frequency magnetic field B1 which is nearly perpendicular to the static magnetic field B0, is focused into a formation traversed by the wellbore, and is superimposed on the static field B0 within the formation disposed opposite the magnetic housing. The frequency of the B1 field is variable so that the tool can be tuned over a wide frequency interval. The CMRT tool makes a measurement in the formation by magnetically reorienting the nuclear spins of particles in the formation with pulses of the magnetic field B1 and then detecting, via the antenna, the precession of the tipped particles in the static magnetic field B0 within the formation. A series of results are then transmitted uphole for processing in a well truck computer.
However, during the measurement, magnetic particles disposed in the wellbore (or "junk"), potentially resulting from the original drilling of the wellbore, are magnetically attracted to and accumulate on the magnet housing which encloses the magnet array. This accumulation of junk on the magnet housing causes an error to be inherently incorporated into the results of the measurement. As a result, when the results of the measurement are transmitted uphole, erroneous results are recorded on an output record medium.
As a result, a method and apparatus is needed for calibrating or tuning the CMRT tool prior to taking a measurement when the tool is disposed in the wellbore, the calibration or tuning procedure being designed to take into account the accumulation of the magnetic junk on the magnet housing of the CMRT tool.